Monocrystalline and polycrystalline silicon wafers are the base material to produce to manufacture monocrystalline and polycrystalline solar cells, respectively. The production process to manufacture a solar cell from a raw wafer includes various steps, and the costs to manufacture a cell from a raw wafer are approximately as high as the material costs for the wafer. Therefore it is important that only wafers that are free from defects are used in production. Critical defects are penetrating and non-penetrating micro cracks, pinholes and inclusions. Micro cracks can cause breakage of the wafer or solar cell during or after production. After production the solar cell may break while mounted to a solar module or cause failure of a mounted or operating module following warranty claims.
Micro cracks are hard to detect within the polycrystalline structure of a wafer because they resemble the crystalline borders. The cracks are then detected by advanced image processing methods. DE 101 46 879 applies an area scan to take the inspection images. A line scan setup is disclosed in EP 1 801 569.
More recent approaches try to capture inspection images where the polycrystalline structure of the wafer is attenuated while maintaining the contrast of the cracks.
The US-Patent application US 2011/058161 discloses a method for detecting defects in an object. The object is locally illuminated by radiating in light having a wavelength to which the object is transparent. Multiple reflected components of the incident light are detected while the detection of directly transmitted components of the incident light is at least partly avoided and the detection of singly reflected components of the incident light is at least partly avoided. Defects are identified by evaluating intensity differences in the detected components of the incident light.
Methods, apparatuses or set-ups which use the image acquisition that captures the directly transmitted light and then finds the cracks by advanced image processing methods suffer from over or under kill. The reason is that the present defect detection image processing methods are not reliable enough due to the presence of grain boundaries in the polycrystalline silicon images.